From 1940 to 1970, the prime requirement for electronic identification systems was to separate friends from foes so that foes could be attacked without endangering friends. Since most weapons had little or no capability beyond line-of-sight range, most identification systems were designed to operate only within this range, and all were designed to make identifications quickly so that attacks could be timely. Identification information has been relayed from one unit to another in some cases to provide advance warning over greater than line-of-sight ranges. The use of satellites to perform communications and navigation functions, and their potential use for weaponry now expands the identification problem. Both greater ranges and new functions beyond those ordinarily considered are important.
The military has been concerned with the identification of remote vehicles (primarily aircraft and ships) for over 30 years. Primary emphasis has been on identification by interrogation-reply systems, and usually these have been associated with radars so that the targets detected by radar could be identified. The possibility of using a time-division data link to identify aircraft was investigated by the Air Force in the early 1950's, but was finally abandoned to permit concentration on interrogation-reply type systems. Previous, present and planned military identification systems have made use of interrogation reply techniques, or have required precise time synchronization or have been vulnerable to enemy attempts to appear as friends, i.e., to enemy "spoofing."
The Mark XII Identification System and its predecessors have been concerned only with the identification of vehicles that are within direct, or "line-of-sight," range. The rapidity with which such identifications usually have to be made makes this category of identification requirements the most difficult to satisfy with a one-way reporting-type system because of the large number of reports and the speed with which data must be processed.
It became clear in the late 1950's that the sophistication required to make the Mark XII system truly effective made it essentially a very special type of digital communications system. Somewhat similar digital communications systems had also been developed by that time for navigation purposes, and a large variety of such systems had been developed for communications functions. In recognition of the similarities, and in some cases duplication, of functions to be performed, it was suggested that the cooperative functions in communications, navigation, and identification systems should be combined.